Optical recording is most common format employed for analog motion picture sound tracks. This analog format uses a variable area method where illumination from a calibrated light source is passed through a shutter modulated with the audio signal. The shutter opens in proportion to the intensity or level of the audio signal and results in the illumination beam from the light source being modulated in width. This varying width illumination is directed to expose a monochromatic photographic film which when processed, for example, results in a black audio waveform envelope surrounded at the waveform extremities by a substantially clear or colored film base material. In this way the instantaneous audio signal amplitude is represented by the width of the exposed and developed film track. FIG. 1 depicts in greatly simplified form an arrangement for recording a variable width analog audio sound track.
A second method can be employed for analog motion picture soundtracks where the audio signal causes the total width of the photographic audio track to be variably exposed. In this method, termed variable density, the exposure of the complete track width is varied in accordance with the intensity of the audio signal to produce a track which varies transmission, for example, between substantially clear or colored base film material and low transmission or high density areas of exposed and developed photographic material. Thus the instantaneous audio signal amplitude is represented by a variation in the transmission of illumination though the exposed and developed film track width.
Hence with either variable density or variable area recording methods the audio modulation (sound) can be recovered by suitably gathering, for example by means of a photo detector, illumination transmitted through the sound track area.
These analog film sound recording techniques can be subject to imperfections, physical damage and contamination during recording, printing and subsequent handling. Since these recording techniques use photographic film, the amount of light used in recording (Density) and the exposure time (Exposure) are critical parameters. The correct density for recording can be determined by a series of tests to determine the highest possible contrast whilst maintaining a minimized image spread distortion.
Image spread distortion results when a spurious fringing image is produced beyond the outline of the wanted image. Typically image spread distortion results from diffusion of light within the film base, between the halide grains and the surrounding gelatin. This scattering of light causes an image to be formed just beyond the exposed area. Optimal negative and positive density and exposure can yield a clean sharp well defined image. However, with variable area recorded negatives, image spreading causes the peaks of the audio modulation envelope appear to be rounded while the valleys of the envelope appear to be sharpened and decreased in width. This image distortion causes a non-symmetrical envelope distortion which translates into both odd harmonic distortion and cross modulation distortion in the recovered audio. As the recording density is increased the image spreading increases and consequently becomes evident as sibilance, initially in the higher frequency content, because of the shorter recorded wavelengths. Increasing the recording density further, causes the distortion to become noticeable at progressively lower frequencies in the recorded spectrum.
Sound recording film is generally only sensitive blue illumination and employs a gray anti-halation dye to substantially reduce or eliminate halation effects. Halation can result from reflections from the back of the film base causing a secondary, unwanted exposure of the emulsion. Typically a fine grain and high contrast emulsion is used with a control gamma between 3.0 and 3.2.
The frequency response of these recording methods is determined by various parameters, for example, the speed at which the shutters open and close, the exposure of the film, and the modulation transfer function MTF of the film which is directly related to light diffusion. The higher the exposure time the lower the frequency bandwidth of the recording.
With these optical recording methods the resulting audio signal to noise ratio can be optimized by use of a high contrast image. For example, the darker audio envelope waveshape and the clearer the surroundings, the cleaner or quieter will be the sound. However, there is a limitation to the possible density at which the film can be exposed at without introducing audio distortion due to image spreading in the film emulsion.
Optimum density presents a compromise between signal to noise ratio and image spread distortion. An optimum density can be determined by test exposures to find an acceptably low value for cross modulation distortion resulting from image spreading. Frequently older or archival audio tracks are improperly recorded and can exhibit severe distortion. However, often some image spread distortion is tolerated in order to obtain an improved audio signal to noise ratio. FIG. 2 shows a somewhat complementary variation of cross modulation distortion with density when printing from negative to positive film sound stock.
In addition to density and image spread distortion other imperfections can result, for example the density of the exposed or unexposed areas can vary randomly or in sections across or along the sound track area. During audio track playback such density variations can directly translate into spurious noise components interspersed with the wanted audio signal.
A further source of audio track degradation relates to mechanical imperfections variously imparted to the film and or it's reproduction. One such deficiency causes the film, or tracks thereon, to weave or move laterally with respect to a fixed transducer. Film weave can result in various forms of imperfection such as amplitude and phase modulation of the reproduced audio signal.
Analog optical recording methods are inherently susceptible to physical damage and contamination during handling. For example, dirt or dust can introduce transient, random noise events. Similarly scratches in either the exposed or unexposed areas can alter the optical transmission properties of the sound track and cause sever transient noise spikes. Furthermore other physical or mechanical consequences, such as the film perforation, improper film path lacing or related film damage can introduce unwanted cyclical repetitive effects into the soundtrack. These cyclical variations can introduce spurious illumination and give rise to a low frequency buzz, for example having an approximately 96 Hz rectangular pulse waveform, rich in harmonics and interspersed with the wanted audio signal. Similarly picture area tight leakage into the sound track area can also cause image related audio degradation.
A German application DE 197 29 201 A1 discloses a telecine which scans optically recorded sound tracks. The disclosed apparatus scans the sound information signal and applies two dimensional filtering to the output values. A further German application DE 197 33 528 A1 describes a system for stereo sound signals. An evaluation circuit is utilized to provide only the left or the right sound signal or the sum signal of both as a monophonic output signal.
Clearly an arrangement is needed that allows optically recorded analog audio sound tracks to reproduced and processed to not only substantially eliminate the noted deficiencies but to enhance the quality of the reproduced audio signal.